1. Technical Field
The present invention relates to a tunable filter, a duplexer, and a communication apparatus, using a semi-coaxial cavity resonator.
2. Background Art
A filter allowing a center frequency thereof to be varied to a desired value according to an external control signal, is called a tunable filter. A typical example of the tunable filter may be a tuning frequency variable band pass filter as disclosed in Japanese Unexamined Patent Application Publication No. 9-284097. The filter has a plurality of short-circuit bars. Ends of the short-circuit bars are respectively fixed at a predetermined height on a lateral surface of an inner conductor bar of a semi-coaxial cavity resonator to be equally spaced, extended radially in the horizontal direction, and then bent substantially perpendicularly downward. Other ends of the short-circuit bars reach a bottom surface of an outer conductor in a circle concentric to a fixed position of the inner conductor, and are respectively connected to PIN diodes fixed on the bottom surface of the outer conductor. The PIN diodes allow the short-circuit bars to be electrically connected to/disconnected from the bottom surface of the outer conductor by applying an external control voltage, so as to control frequencies of respective resonators according to variation in inductance components due to the shorting of the short-circuit bars relative to the outer conductor, and thus, to allow a tuning frequency of the filter to be varied.
In the above-described configuration, since a plurality of ground lines continuously arranged with the PIN diodes are provided in the semi-coaxial cavity resonator, at a portion to which extremely high current is applied, the Q of the resonator may decrease, and an insertion loss may increase in a band pass filter or a band elimination filter, in which the resonators are continuously arranged.
In addition, in this configuration, since frequencies are varied according to the on/off state of the PIN diodes, a center frequency of the band pass filteror band elimination filter is varied only discretely, and may not be varied steplessly.
Also, when a filter needs to pass electricity as high as several watts to several tens of watts, a high voltage might be applied to the PIN diodes. Accordingly, the PIN diodes may be burned out, and the filter may be no longer usable.
Similarly, when the filter needs to pass electricity as high as several watts to several tens of watts, the PIN diodes, and the combination of the ground lines and the PIN diodes may cause a high level of intermodulation to occur.
To solve the above-described problems, embodiments of the disclosed invention may be configured as follows:
According to a first aspect of the invention, a tunable multi-stage semi-coaxial cavity band pass filter, in which adjacent stages are electromagnetically coupled, ,may include: an outer conductor having inside a plurality of separate cavities divided by partitions; a rod-like inner conductor fixed on a bottom surface of each cavity, but not fixed on a surface which faces the bottom surface of each cavity; a frequency-adjusting screw made from a conductor and screwed through the surface which faces the bottom surface of each cavity, or through a lateral surface of each cavity; an input/output connector attached to the outer conductor; and a coupling slit provided at each partition provided between the adjacent cavities, in which adjacent stages are electromagnetically coupled; in which in each cavity, a dielectric held by a holder, which is movably inserted through the outer conductor, is inserted into each cavity, and a plurality of the holders have respective projecting portions which project out of the outer conductor, and the plurality of projecting portions are coupled by a coupling member.
According to a second aspect of the invention, a tunable semi-coaxial cavity band elimination filter may include: an outer conductor having inside a plurality of separate cavities divided by partitions; a rod-like inner conductor fixed on a bottom surface of each cavity, but not fixed on a surface which faces the bottom surface of each cavity; a frequency-adjusting screw made from a conductor and screwed through the surface which faces the bottom surface of each cavity, or through a lateral surface of each cavity; a transmission line having an input/output unit attached to the outer conductor; and a connector which electrically connects the transmission line to a resonating electromagnetic field occurring at each cavity; in which in each cavity, a dielectric held by a holder, which is movably inserted through the outer conductor, is inserted into each cavity, and a plurality of the holders have respective projecting portions which project out of the outer conductor, and the plurality of projecting portions are coupled by a coupling member.
Referring to the tunable band pass filter according to the first aspect of the invention, in a modification according to a third aspect of the invention, a movable region of the dielectric disposed in a predetermined cavity is a region other than a region defined by a bottom surface, which is an opening portion of the slit provided in the predetermined cavity, and an apex, which is an arbitrary point on the central axis of the inner conductor disposed in the predetermined cavity.
Referring to the tunable band pass filter according to the first or third aspect of the invention, a modification according to a fourth aspect of the invention further includes a mechanism for individually determining an insertion amount of each dielectric inserted into each cavity.
Referring to the tunable band elimination filter according to the second aspect of the invention, a modification according to a fifth aspect of the invention further includes a mechanism for individually determining an insertion amount of each dielectric inserted into each cavity.
According to a sixth aspect of the invention a duplexer includes: at least two filters; and an antenna connector connected to the filters in a shared manner, in which at least one of the filters is the tunable band pass filter according to any one of the first, third and fourth aspects of the invention.
According to a seventh aspect of the invention a communication apparatus includes: the duplexer according to the sixth aspect, a transmission circuit connected to at least one of the input/output connectors of the duplexer; a reception circuit connected to at least another one of the input/output connectors; and optionally, an antenna connected to the antenna connector of the duplexer.
With the first or second aspect of the invention, when the position, the angle, or both, of the dielectric inserted into the resonant cavity of each semi-coaxial cavity resonator with respect to the inner conductor is varied, perturbation is applied to an electric field occurring in the resonant cavity, so that the resonant frequency of each cavity resonator may be varied. In addition, since the holders of the dielectrics are coupled by the coupling member, the frequencies of the resonators are varied immediately at the same time, and by the same variation amount. Since the displacement amount of the coupling member is arbitrarily set within a movable range, there is provided a tunable filter that allows the center frequency of the band pass filter or the center frequency of an elimination band of the band elimination filter to be arbitrarily set within the movable range, and has good responsiveness.
Further, by movably coupling the holders to the coupling member, the perturbation amount may be varied in each of the resonators by way of a cam structure or the like. Owing to this, even if the filter is configured by continuously arranging resonators having resonant cavities with different profiles , the displacement amount of each of the dielectrics may be individually varied among the resonators. Therefore, since the variation amounts of the frequencies may be arbitrarily set for the resonators, respectively, with the tunable filter, deterioration of the filter characteristics may be restrained, and the center frequency may be freely selected.
Since the perturbation due to the dielectric is applied to the electric field in the resonant cavity, the decrease in Q of the resonator may be minimized by using a dielectric having a good dielectric loss tangent. In addition, since it is not necessary to use an active element such as a PIN diode in a resonant cavity to which extremely high current is applied, power resistance characteristics of the filter can be enhanced, thereby increasing reliability. Further, since it would be unnecessary to attach an excessive component like a PIN diode or a ground line combined with a PIN diode to the filter, there is provided a tunable filter in which intermodulation due to such excessive component does not occur.
With the third aspect of the invention, since the perturbation due to the displacement of the dielectric bar is not applied to the region where the adjacent resonators are electromagnetically coupled, the variation in relation to the coupling coefficients provided between the stages is suppressed while only the frequencies are varied. Accordingly, there is provided a tunable band pass filter in which a return loss waveform is less disordered.
With the fourth and fifth aspects of the invention, the insertion amounts, with respect to the cavities, of the dielectrics which allow the frequencies to be varied, can be determined appropriately. Accordingly, even when the filter has resonators having different cavity profiles or resonators having different sensitivities of the perturbation of the resonant frequencies with respect to the positional displacement of the dielectrics, the insertion amounts of the dielectrics may be pre-adjusted such that the perturbation amounts of the resonant frequencies become the same. In addition, the mechanism can be applied to fine adjustment of the frequencies during adjustment of the filter.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings.